Do I Need a 3D Player App on Meta Quest?

No. The fastest way is the Quest browser: open your video and press "Enter VR". Nothing to install.

You only need a player app if you downloaded the file and want to play it from the headset itself.

If you do, these work well:

• Skybox VR, our first choice. It detects Full SBS reliably.
• DeoVR, free, with strong codec support.
• Pigasus, a good alternative with network streaming.

In any of them, set the mode to "Side by Side" and "3D".